1. Field
The following disclosure relates to an operational amplifier including an overdriving circuit and, more particularly, to an operational amplifier including an overdriving circuit capable of reaching a target voltage within an operation time by outputting a higher voltage than the target voltage when an RC delay time is greater.
2. Description of the Related Art
In general, an operational amplifier, having two input terminals and one output terminal, is an amplifier for amplifying the difference between voltages of the two input terminals. The input terminal of the operational amplifier is configured as a differential amplifier, and because a circuit for performing the four fundamental arithmetic operations can be configured by using the operational amplifier, it is called an “operational amplifier” in the sense of an operator. In actuality, the operational amplifier has a very high voltage gain and input impedance and very small output impedance.
FIG. 1 illustrates a general operational amplifier, and FIG. 2 is a timing diagram of the operational amplifier.
With reference to FIGS. 1 and 2, an input voltage Vin is applied to an input terminal (+) of an operational amplifier 10, and an input terminal (−) is connected to an output terminal Vout. A resistor Rd and a capacitor Cd is connected to the output terminal Vout. When an input voltage Vin having a certain level is applied to the input terminal (+), the input voltage Vin, which has been amplified to have a certain level, is outputted from the output terminal Vout.
The operational amplifier 10 may be applicable for a data driving chip of a liquid crystal display (LCD), and in this case, the operational amplifier 10 is included in an output terminal of the data driving chip. In the data driving chip, the operational amplifier 10 is used as an analog buffer that outputs an inputted voltage to a data line, and in this case, the data line includes a high load, for example, the resistor Rd and the capacitor Cd, in view of an equivalent circuit.
It is designed such that the voltage of the operational amplifier has a sufficiently high slew rate to allow the voltage of an output terminal Cout of the capacitor to reach a target voltage Vt within an operation time T. Here, the slew rate is a thermometer that denotes how fast an output voltage is with respect to an input voltage, which is also called an “increase rate” of an output voltage.
In this case, however, when an RC delay time is relatively large while the operation time T is relatively small because the values of the resistor Rd and the capacitor Cd connected to the output terminal of the operational amplifier are great, no matter how fast the slew rate of the operational amplifier is, the RC delay time is large, so the voltage of the output terminal Cout cannot reach the target voltage Vt within the determined operation time T.